1. Field of the Invention
This invention relates to a method for preparing an optical disk master for use in the manufacture of optical disk substrates.
2. Prior Art
Optical disks include write-once read many or rewritable optical recording disks and read-only disks. The optical recording disks have recording layers formed on disk substrates which are provided on the surface with (guide) grooves for tracking and other purposes. In the read-only disks, information-bearing pits are integrally formed on the surface of disk substrates.
Disk substrates are usually prepared by injection molding resins using a stamper having a negative pit and groove pattern. The stamper is often formed of nickel. In order to manufacture the stamper, an optical disk master is first prepared. The following process is generally employed for preparing the optical disk master. First, a photoresist layer is applied on the surface of a glass substrate. The photoresist layer is exposed to laser beams to form a latent image of the desired pattern, followed by development.
Using the optical disk master, a stamper is manufactured usually by the following process. First, to impart electric conductivity to the surface of the photoresist layer on the optical disk master, a metal thin film of nickel or the like is formed as by sputtering or electroless plating. Electroforming is then effected to deposit a film of nickel or the like on the metal thin film. Then the laminate of the metal thin film and the electroformed film is stripped from the photoresist layer. The laminate is ready for use as the stamper master. This stamper master may be used as the stamper directly, although a stamper mother may be prepared from the stamper master and used as the stamper. The stamper mother is prepared by electroforming a film on the surface of the stamper master and striping the electroformed film. It is recommended to previously oxidize the surface of the stamper master so that the electroformed film may be readily stripped therefrom. Alternatively, a stamper child may be similarly prepared using the stamper mother and used as the stamper.
In the process of preparing an optical disk master, the minimum width of the latent image pattern formed in the photoresist layer is limited by the diameter of a laser beam spot. The beam spot diameter w is represented by w=kxc2x7xcex/NA wherein xcex is the wavelength of the laser beam, NA is the numerical aperture of an objective lens in an optical system, and k is a constant which is determined by the aperture shape of the objective lens and the intensity distribution of an incident light flux. Therefore, to reduce the pit or groove width, the wavelength of the laser beam must be decreased or the numerical aperture of the objective lens be increased.
However, since an objective lens having a numerical aperture in excess of 0.9 has been already developed and used in practice, there is left little room for further improvement. As to the laser wavelength, a blue laser of short wavelength has been used in practice. Then, to further reduce the minimum width of a latent image pattern, a laser beam of a shorter wavelength, for example, in the ultraviolet region must be used. On use of a laser beam in the ultraviolet region, however, optical components including the objective lens should be changed to those capable of complying with the ultraviolet region. This gives rise to a serious problem that a substantial modification of the system is needed.
JP-A 1-317241 proposes a process of preparing an optical disk master wherein the steps of immersing an unexposed photoresist film in a developer or showering the developer to the unexposed photoresist film, followed by water washing and drying are added prior to the step of exposing the photoresist film to radiation. By adding the developing, washing and drying steps prior to the exposure step, the photoresist film surface is substantially insolubilized, allowing grooves or pits to be formed to a narrower width or smaller diameter. In this patent, the development prior to exposure is effected at a concentration of 50% for a time duration of 10 seconds.
JP-A 6-260407 discloses a process for forming a resist pattern by applying a photoresist layer onto a semiconductor substrate, treating the photoresist layer with an alkali to form an insolubilized stratum in the surface of the photoresist layer, followed by exposure and development. The mechanism of this process is as follows. As a result of the alkali treatment, low molecular weight components in the photoresist are dissolved out under the action of alkali, whereby the surface of the photoresist layer is hardened, and at the same time, the alkali causes azo-coupling reaction to take place between the novolac resin and photosensitive groups in the photoresist to form an insolubilized stratum in the photoresist layer surface. This insolubilized stratum holds down the dissolution rate in the unexposed areas of the photoresist layer, resulting in an improvement in the resolution of the positive acting photoresist. In the patent, the alkali treatment is effected by building up a quaternary ammonium hydroxide solution on the photoresist layer.
In the above-referred JP-A 1-317241 and 6-260407, the photoresist layer is substantially insolubilized by treating with a developer or quaternary ammonium hydroxide solution, thereby achieving an improved resolution. Such alkaline solutions as the developer and quaternary ammonium hydroxide solution need safe disposal as by neutralization, inviting a cost increase.
The photoresist becomes alkali soluble upon exposure. Therefore, the alkali treatment must be done prior to exposure as described in the above-referred patents. The alkali treatment is carried out by immersion in alkaline solution or by showering of alkaline solution, which must inevitably be followed by washing with pure water and drying. Since the photolithographic system of carrying out a series of steps in an in-line mode does not usually have washing and drying units incorporated between the photoresist applying step and the exposure step, the alkali treatment must be carried out off-line.
An object of the invention is to provide a method for preparing an optical disk master having a finer pattern at a reduced cost and with a minimal burden on the manufacturing system.
The invention is directed to a method for preparing an optical disk master having a protrusion/depression pattern on its surface, comprising the steps of applying a photoresist layer on a substrate, exposing the photoresist layer to radiation to form a latent image of the protrusion/depression pattern, and developing the latent image. According to the invention, the step of treating the photoresist layer with a surfactant is provided prior to the exposing step or between the exposing step and the developing step. The treating step is preferably effected between the exposing step and the developing step. The treating step preferably includes bringing a solution of the surfactant in contact with the photoresist layer by spin rinsing or dipping. Typically the surfactant is a nonionic surfactant. Preferably the treating step uses a surfactant solution having a concentration of 0.1 to 10% by weight of the surfactant.
The exposing step is preferably to expose the photoresist layer to a dose of radiation D1 which satisfies the relationship: 1.1xe2x89xa6D1/D0xe2x89xa62 wherein D0 is the optimum exposure dose for the photoresist layer when the treating step is excluded.